This invention relates to electroluminescent devices and, in particular, to an electroluminescent device which uses a low electron affinity electron conductor as the electron transport device to facilitate the generation of light.
Electroluminescent devices can be used in display applications interchangeably with liquid crystal devices ("LCDs") and cathode ray tubes ("CRTs"), among others. Applications for electroluminescent devices, LCDs and CRTs are expanding and include medical, consumer, and communication applications to name a few. However, electroluminescent devices have cornered only a small share of this expanding market. This is partly due to various deficiencies of the electroluminescent devices currently on the market.
One type of electroluminescent device uses impact ionization to cause luminescence within an electronic material. In general, these devices apply an electron voltage to induce a photon emission from the electronic material, which can include a host solid, such as ZnO or ZnS, and metals placed within a host lattice, such as rare earth metals. These devices generate a large quantity of heat, and thus have a short life time. Another type of electroluminescent device uses avalanche excitation and tunnel injection mechanisms to cause the luminescence. These devices demonstrate similar limitations. All these devices also have high power requirements.
Another type of electroluminescent device that uses inorganic elements relies on injecting minority carriers into a solid state material, such as GaAs or GaP, to cause recombination in the majority carrier region and the emission of a photon. The color spectrum that these devices can emit is usually limited to the band-gap of the solid. Although the introduction of GaN has extended the emissions of these electroluminescent devices to the blue portion of the spectrum, the electroluminescent devices require quantum wells for fabrication to increase the energy of the emitted light. Thus, the devices that can emit colors from all portions of the visible spectrum are expensive to fabricate.
Organic electroluminescent devices have several advantages over the inorganic devices in that they are easier to fabricate and have a greater efficiency by an order of magnitude. Organic electroluminescent devices also require lower voltages and can emit light throughout the visible spectrum. The cathode element of the organic electroluminescent devices still exhibits several problems. The cathode is usually fabricated from a substance that has a low work function, such as alkaline metals and alkaline earth metals, for example. However, many metals spontaneously oxidize when exposed to oxygen. A thin layer of the oxide is formed on a surface of the cathode, usually on the surface juxtaposed to the electroluminescent device. The oxidation diminishes the amount of free electrons available to be transported into the electroluminescent device.
Moreover, the electron mobility of the electron conductor in the organic electroluminescent device is fairly low. Thus, the organic devices, while exhibiting a higher efficiency overall, have poorer electron transport characteristics when contrasted with an electroluminescent devices that has an inorganic electron conductor.
Another problem associated with an electron transport material in the organic electroluminescent device is that it generates heat when an electric field is created across the device. Heat results in a loss of efficiency and long term degradation of the electroluminescent device. Efficiency is decreased because any energy released as heat cannot then be used to generate luminescence. In addition, the resistivity of the electron transport material will increase with the increase in temperature and not conduct the electrons as efficiently. Further, the heat leads to long term degradation of the device owing to creation of short circuits through the conducting organic elements, creation of gas pockets formed at the metal-organic interface caused by the loss of adhesion of the organic substance to the metal substance acting as the cathode or electron transport material, and crystallization of any hole conducting molecules.
One approach to solving the stability problem associated with a cathode fabricated from metal involves using an alloyed metal as the cathode. While this may improve the oxidation problems, the electroluminescent device still suffers from stability problems due to fabricating the cathode adjacent to organic materials. The difficulty of fabricating the electroluminescent device is increased when the cathode is a crystalline metal. Moreover, such devices usually require a permanent seal to guard against degradation. Thus, such devices become even more costly to fabricate and still are not very robust.
Accordingly it is an object of this invention to provide an electroluminescent device that is more stable and has a longer life time.
It is also an object of this invention to provide an electroluminescent device that has lower power requirements.
It is a further object of this invention to provide an electroluminescent device that decreases the generation of heat.
It is another object of this invention to provide an electroluminescent device that can be fabricated using less expensive fabrication techniques while having a high efficiency.
It is another object of the invention to provide an electroluminescent device that has the efficiency characteristics of an organic electroluminescent device or better while providing an electron conductor that has the mobility more like that of an inorganic electron conductor.
It is another object of this invention to provide an electroluminescent device with a low work function cathode.
These and other objects of the invention will be obvious and will appear hereinafter.